1. Field of the Invention
The present invention relates to a wire bonder for electrically bonding chips together by a bonding wire, for example, in a multichip package, and a wire bonding method using the wire bonder.
2. Description of the Related Art
In a multichip package, a plurality of chips is sealed in one package. This type of multichip package is disclosed in U.S. Pat. No. 5,126,823. In the multichip package, there is a case where a pad formed on a chip is bonded to a pad formed on another chip by a bonding wire. To electrically bond different chips together, a wire bonding method using ball bonding and wedge bonding is known. This method is described in the paragraph of "Prior Art" of Published Unexamined Japanese Patent Application No. 2-114545. The method includes first and second bonding steps. The first bonding step is executed by the ball bonding in which a wire is bonded to a bonding pad on a first semiconductor chip using a metal ball formed at the end of a capillary. The second bonding step is executed by the wedge bonding in which the wire extended from the first semiconductor chip, which is formed line a loop, is bonded to a bonding pad on a second semiconductor chip, and then cut.
A method of forming a metal ball on the bonding pad on the second semiconductor chip in advance in the second bonding step, is disclosed in Published Unexamined Japanese Patent Applications Nos. 2-114545 and 60-41236. According to this method, the metal ball is previously formed on the bonding pad (second bonding point) on the second semiconductor chip before the first bonding step. After the first bonding step, the wire extended from a first bonding point is bonded to the metal ball by the wedge bonding. When the metal ball is formed at the second bonding point, a wire cutting point has to be located at a predetermined distance d1 from the second bonding point. Further, when the wire extended from the first bonding point is bonded to the metal ball, a wire cutting point has to be located at a predetermined distance d2 (d2&gt;d1) from the second bonding point.
Conventionally, an operator of the wire bonder calculates coordinates of the first and second bonding points and distances d1 and d2 (coordinates of the wire cutting points) and inputs the calculated coordinates to the wire bonder as data of bonding.
In the aforementioned bonding method, however, if the mount locations of the first and second semiconductor chips are shifted from each other, the operator has to correct the coordinates of the first and second bonding points and those of the wire cutting points to input them to the wire bonder. For this reason, the calculation of the coordinates is complicated when a plurality of wires is bonded to semiconductor chips. When bonding is executed, the facing sides of two semiconductor chips do not often cross a wire extending from the first bonding point at right angles. If the mount locations of the chips are shifted from each other, both a bonding point of a metal ball and a wire cutting point are shifted from the extended wire. This shift deforms a loop of the wire, that is, an under loop or a loop curl is formed. Therefore, the wire contacts the semiconductor chips to cause a short circuit or the wires contact each other, causing a defective chip.